1. Technical Field
The present disclosure relates to a spectrometer including a Geiger-mode avalanche photodiode.
2. Description of the Related Art
In the technical field of photon detection, so-called Geiger-mode avalanche photodiodes (GMAPs) are known, which can theoretically enable detection of single photons.
A Geiger-mode avalanche photodiode, also known as single-photon avalanche diode (SPAD), is formed by an avalanche photodiode (APD), and hence comprises a junction of semiconductor material, which has a breakdown voltage VB and is biased, in use, with a reverse-biasing voltage VA higher in modulus than the breakdown voltage VB, which, as is known, depends upon the semiconductor material and the doping level of the least doped region of the junction itself. In this way, the junction has a particularly extensive depleted region, present inside which is a non-negligible electrical field. Hence, generation of a single electron-hole pair, caused by absorption within the depleted region of a photon incident on the SPAD, can be sufficient to trigger a process of ionization. This process of ionization in turn causes an avalanche multiplication of the charge carriers, with gains in the region of 106, and consequent generation in short time intervals (hundreds of picoseconds) of the so-called avalanche current, or more precisely of a pulse of the avalanche current.
The avalanche current can be collected, typically by means of an external circuitry connected to the junction, for example, by means of appropriate anode and cathode electrodes, and represents an output signal of the SPAD, also referred to as output current. In practice, for each photon absorbed, a pulse of the output current of the SPAD is generated.
The fact that the reverse-biasing voltage VA is appreciably higher than the breakdown voltage VB means that the process of avalanche ionization, once triggered, is self-sustaining Consequently, once triggered, the SPAD is no longer able to detect photons, with the consequence that, in the absence of appropriate remedies, the SPAD manages to detect arrival of a first photon, but not arrival of subsequent photons.
To be able to detect also the subsequent photons, it is necessary to turn off the avalanche current generated within the SPAD, arresting the process of avalanche ionization, and in particular lowering, for a period of time known as hold-off time, the effective voltage Ve across the junction so as to inhibit the ionization process. For this purpose, it is known to use so-called quenching circuits, whether of an active or passive type. Next, the reverse-biasing voltage VA is restored in order to enable detection of a subsequent photon.